Background generation caused the UI to freeze indefinitely when
switching LUT files, likely due to a deadlock between the worker
process, the preview autocacher, and the asynchronous set_processor
path. Synchronous generation is fast enough for typical 33^3 .cube
files and keeps the cache invalidation logic simple and safe.
The passthrough fix in OCIOBaseNode::Value() is retained so the
viewer shows the input frame while a processor is being created.